Gas sensors are employed in a variety of applications requiring qualitative and quantitative gaseous determinations. In the automotive industry, it is well known that the oxygen concentration in the automobile exhaust has a direct relationship to the engine air-to-fuel ratio. Oxygen gas sensors are employed within the automobile internal combustion control system to provide accurate exhaust gas oxygen concentration measurements for determination of optimum combustion conditions, maximization of efficient fuel usage, and management of exhaust emissions.
Typically, the electrochemical type of oxygen sensor employed in automotive applications utilizes a thimble-shaped electrochemical galvanic cell to determine, or sense, the relative amounts of oxygen present in the exhaust stream, as disclosed in U.S. Pat. No. 3,844,920 to Burgett et al. This type of oxygen sensor comprises an ionically conductive solid electrolyte material, typically yttria stabilized zirconia, a porous electrode coating on the exterior exposed to the exhaust or measuring gas and a porous electrode coating on the interior exposed to a known concentration of reference gas. The gas concentration gradient across the solid electrolyte produces a galvanic potential which is related to the differential of the partial pressures of the gas at the two electrodes by the Nernst equation: E=AT ln[P.sub.1 /P.sub.2 ], where E is the galvanic voltage, T is the absolute temperature of the gas, P.sub.1 /P.sub.2 is the ratio of the partial pressures of the reference gas at the two electrodes, and A=R/4F, where R is the universal gas constant and F is the Faraday constant.
Currently, these oxygen sensors are employed in the exhaust gas system of an internal combustion engine to determine qualitatively whether the engine is operating at either of two conditions: (1) a fuel rich or (2) a fuel lean condition, as compared to stoichiometry. After equilibration, the exhaust gases from these two operating conditions have two widely different oxygen partial pressures. This information is provided to an air-to-fuel ratio control system, so that it can provide an average stoichiometric air-to-fuel ratio between the two conditions. However, due to increasing demands for improved fuel utilization and emissions control, it is desirable to operate internal combustion engines exclusively within lean combustion conditions, i.e., air-to-fuel ratio between 15:1 and 25:1, where changes in the after-combustion oxygen partial pressures are only gradual and slight. The current oxygen sensor is not sensitive enough for this latter type of environment.
To be an effective component of the internal combustion control system operating exclusively within lean combustion conditions, the oxygen sensor must be extremely sensitive and capable of rapid, precise, absolute oxygen concentration measurements. It is desirable that the sensor must have a response time of less than 0.1 second at a minimum temperature of 300.degree. C. and a maximum oxygen concentration at the sensing electrode of about eight percent. To prevent false sensor readings, the sensor should be hermetically sealed, and must also be free from any current leakage caused by electronic conduction in the solid electrolyte body. The sensor must be structurally sound to withstand the considerable vibration and wide temperatures ranges, -40.degree. C. to 800.degree. C., that it may be exposed to. Most importantly, the sensor should be amenable to mass production.
Internal reference oxygen sensors have been devised for lean engine operation and typically comprise two solid electrolyte galvanic cells: the first galvanic cell senses the gas to be measured, while the second galvanic cell generates an accurately known internal gas reference. The accurately known internal gas reference is generated by electrochemically pumping oxygen gas into and out of a hermetically sealed, fixed volume chamber by means of the second galvanic cell. An external power source provides a potential across the solid electrolyte body of the second galvanic cell. Electrons supplied at one electrode ionize gas molecules at the interface between that negatively biased electrode and the solid electrolyte. The gas ions are then transported through the solid electrolyte by ionic conduction. At the other electrode, the gas ions lose electrons and recombine into gas molecules. By reversing the polarity of the external circuit, oxygen gas can be transported in the other direction and subsequently pumped out of the hermetically sealed, fixed volume chamber. The partial pressure (i.e., concentration) of oxygen gas in a gas mixture can be measured by simultaneously sensing the oxygen partial pressure differential between the internal reference chamber and the gas mixture with the first galvanic cell.
In the past, internal reference, solid electrolyte gas sensors have generally comprised two discrete galvanic cells bonded within a cylinder or bonded together to form a hermetically sealed, fixed volume chamber. These sensors are difficult and expensive to assemble due to the inherent problems associated with hermetic sealing, and therefore are not suitable for a mass production device. An improvement is disclosed in U.S. Ser. No. 882,689 filed July 7, 1986, now U.S. Pat. No. 4,668,374 by J. K. Bhagat and D. S. Howarth entitled, "Gas Sensor and Method of Fabricating Same" and assigned to the assignee of this invention. The improvement involves laterally positioning both the pump and sense cell components on a single substrate, resulting in a simpler, more efficient, and easier to produce device. In this improvement, pump and sense cells can be disposed in a cavity in the substrate and encased by a hermetically bonded cover plate. Apertures in the substrate expose appropriate electrodes of the pump and sense cells for operation.
Internal reference, solid electrolyte oxygen sensors may be operated in various modes to determine gas concentration measurements. One method is to pump oxygen into the internal reference gas chamber with the pump cell until the pressure therein produces a voltage output at the sense cell that equals zero or a threshold value. The period of time required to pump that amount (i.e., pressure) of oxygen into the reference gas chamber is related to the oxygen partial pressure in the exhaust gas. An alternative method is to maintain a constant oxygen pressure in the internal reference gas chamber and determine exhaust oxygen concentration from the voltage output measurements at the sense cell.
If one elects to cycle oxygen out of and back into the reference chamber each time one chooses to measure oxygen partial pressure in a gas mixture, sensor response time will be proportional to the volume of the internal gas reference chamber, i.e., the number of gas molecules needed to be pumped into and out of the chamber in order to reach a reference pressure. It is desirable to keep the chamber volume to a minimum in order to maximize the quickness of sensor response. Currently, as in the previously mentioned U.S. Ser. No. 882,689, the thickness of the material employed to bond the two substrates comprising the pump and sense cell components limits the minimum chamber volume attainable. Therefore, a solution is to employ a single substrate as the support for both the pump and sense cell components. My invention comprehends a single substrate and uses conventional thin film deposition techniques to produce a chamber of lesser volume.
Sensor response time is also related to the thickness of the solid electrolyte and its ionic resistance. A thin electrolyte film produces a short response time. However, in the aforementioned type of constructions, the electrolyte film must be thick enough to be structurally sound. One typical prior internal reference solid electrolyte oxygen sensor comprises two discrete solid electrolyte concentration cells hermetically sealed together. Each cell is a solid electrolyte disk that is electroded on its opposite faces. Because each solid electrolyte disk provides the entire structural support for the concentration cell, the solid electrolyte disk is accordingly rather thick, and the sensor rather slow in responding. In my invention, we use an alumina substrate to provide support for thin film solid electrolyte layers having thin film porous electrodes.
In the previously filed U.S. Ser. No. 882,689, an internal reference, solid electrolyte oxygen sensor is disclosed comprising thinner solid electrolyte layers than prior internal reference, solid electrolyte sensors. The electrolyte layers are disposed on a substrate in a chamber formed between the substrate and cover member. However, in order to provide access to the exhaust gas at one of the porous electrodes comprised within sense cell, the supporting substrate under the sense cell components must be removed. Subsequently, the solid electrolyte film becomes the structurally supporting member for the porous platinum electrodes. Thus, for strength purposes, a considerable thickness of solid electrolyte material is needed, over that which would be required for electrical performance purposes alone. It is desirable to provide some alternative form of structual support to all components of the oxygen sensor, including the electrolyte layer, so that thicknesses of the component layers can be minimized and sensor response time reduced without any detrimental loss to structural integrity.